Conventional flowmeters measure volumetric flowrate by using two factors; (1) the output frequency, and (2) the K factor which is defined as the calibration constant of the flowmeter. The K factor is determined from calibration data or charts and can be considered as being practically constant only in the range where turbulence is fully developed. As the flow rate decreases to below the turbulent flow regime, the K factor varies significantly and frequency output cannot be used as a direct indication of flowrate. By confining measurements to within the fully developed turbulent flow range, present flowmeters operate only when the Reynolds number is high and thereby circumvent the problem of viscous effects at lower Reynolds numbers.
Large hub diameters and smaller blade lengths have been used to achieve higher Reynolds number per unit (volume) flow. However, the useful flow range of the flowmeter is still restricted, since the bearing thrust also increases rapidly according to the square power of the "boosted-up" flow velocity. Thus, the upper flowrate of the turbine flowmeter is limited by the thrust which the flowmeter bearings can tolerate. Also, the induced high flow velocity may also result in unfavorable annular flow resistance, eddy diffustivity and possibility of cavitation. Even though such measures improve low flow capability of the meter, the lower limit of its measurability is still dictated by the critical Reynolds number, where the transition from turbulence to laminar flow takes place. When magnetic pick-ups are utilized to produce an output frequency, the ability of the flowmeter at low flowrates is further affected by the magnetic coupling which becomes significant relative to the driving torque at lower flowrates. Because of such difficulties as enumerated, most conventional turbulent flowmeters restrict their useful flow range between a narrow span of approximately ten to one. If the flow range to be covered is a wide one, say six hundred to one, several flowmeters must be used. However, the cost of multiple flowmeters with associated additional plumbing and valves, the inconvenience of constantly assembling and disassembling equipment, and the greater complexity of data gathering and processing makes the use of multiple flowmeters undesirable.